(a) Technical Field
The present disclosure generally relates to a catalyst for a fuel cell and a method for preparing the same. More particularly, it relates to a method for preparing a catalyst used for an electrode of a fuel cell by using different types of crystalline carbon materials, and a catalyst for a fuel cell prepared by the method.
(b) Background Art
In general, a fuel cell is a device that converts chemical energy of a fuel, such as hydrogen, into electrical energy. As the theoretical efficiency of fuel cells approaches 100% and generally reaches a high level of 80 to 50%, extensive research has been aimed at more efficiently using fuel cells and renewable energy in the form of hydrogen, particularly as resources become more scarce.
The operation of fuel cells is based on an electrochemical reaction which involves the migration of electrons, and it is important to induce a reaction that can reduce overvoltage such that the polarization at an equilibrium potential is minimized at the same rate of the electrochemical reaction.
For this purpose, the dispersion of catalyst particles should be improved and the catalyst particles should also be provided in a form capable of participating in the reaction.
In general, during operation of the fuel cell, the operating potential is in a range of 1.0 to 0.4 V. In the case of carbon, the thermodynamic standard oxidation potential in gasification is 0.207 VRHE, and thus the natural oxidation will occur a potential higher than 0.207 VRHE.
As such, it has been found that the operating voltage conditions of a fuel cell generates a high oxidation overvoltage with respect to carbon to create a negative atmosphere. Moreover, during start and stop of the fuel cell, outside air is fed into an oxidation electrode (i.e., a fuel electrode) and is mixed with hydrogen as a fuel, and thus a high potential of 1.2 VRHE or higher is generated in the carbon based on its boundary [Electrochem Solid-State Lett. 9 (2006) A183, J. Power Sources 158 (2006) 1306]. Such conditions accelerate the corrosion rate of carbon, and thus the durability of the fuel cell is reduced.
It has, thus, been found that delay of the electrochemical reaction is one important way to potentially improve the durability of a fuel cell.
One proposed method for improving the rate of the electrochemical reaction, which occurs at the catalyst particles of the fuel cell, is by improving the surface reaction rate of the catalyst particles. As such, extensive research has been aimed at developing a platinum alloy catalyst and core/shell type catalyst particles and, at the same time, optimizing the shape of an electrode by controlling the porosity of the electrode, micronizing the catalyst particles, and controlling the effective reaction area (J. Power Sources, 139, 73).
Moreover, in order to improve the durability of the catalyst and the electrode of the fuel cell by delaying the corrosion rate of carbon, extensive research has been aimed at developing a crystalline carbon support with excellent corrosion resistance.
It is reported that typical crystalline carbon nanotubes or nanofibers have a higher oxidation resistance than active carbons such as Ketjen Black and Vulcan because they are difficult to react with external H2O [ECS Trans. 16 (2008) 2101].
However, these high crystalline carbon materials have not been successfully used as a carbon support to fuel cells. It has been found that, micronization of catalyst particles on the surface of a high crystalline carbon support is very difficult, and it is impossible to optimize the electrode structures (e.g. the shape of pores) during electrode formation. For example, in the case where the carbon nanofibers have a small fiber diameter, the straightness of the carbon nanofibers is reduced, and they become entangled with each other. this makes it very difficult to form mesopores in the nanofibers. Moreover, in spite of the high surface area, it is difficult to finely and uniformly disperse catalyst particles, and thus it is difficult to increase the effective active area of the particles. Furthermore, in the case of carbon nanofibers having a large fiber diameter, a high support ratio is not achieved, or the carbon nanofibers must be ground into small particles, which reduces the porosity, thereby reducing the effective specific reaction area.
Linear crystalline carbon materials can be classified into tubular, platelet, and herringbone carbon materials.
Of the three, the tubular carbon materials have the highest oxidation resistance because the base plane is exposed on the surface. However, the tubular carbon material has no space in which the catalyst particles, such as platinum, can be preferably located. As such, it is difficult to achieve a fine and uniform distribution of catalyst particles, and Ostwald ripening occurs due to the surface migration of catalyst particles (Carbon Today, 90, 277).
In the case of the platelet and herringbone carbon materials, the edge of the base plane is exposed on the surface, thereby providing a space in which the carbon particles can be stably supported. However, the oxidation resistance is reduced as a result of exposure of the edge of the base plane on the surface. As such, the materials exhibit an oxidation resistance which is not more advantageous than that of active carbon materials according to circumferences.
Thus, it is necessary to provide a material having the support performance such as that exhibited by platelet or herringbone carbon particles and, at the same time, having the oxidation resistance such as that exhibited by tubular carbon nanofibers.
Further, in order to improve the performance of a fuel cell catalyst, it is necessary to provide an electrode structure in which the triple point of the support for carrying the catalyst and electrons, the ionomer for transporting protons, and the catalyst particles as a reaction site in a three-dimensional shape is properly developed in an area where the material reaction occurs smoothly, i.e., an electrode structure that optimizes such an effective reaction point. Moreover, the development of a material that can ensure the oxidation resistance for ensuring long-term stability is urgently required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.